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ASE P2 Test Prep: Emissions Control System

Information to help parts professonals pass the ASE P2 test.


PCV prevents crankcase blowby vapors from escaping into the atmosphere by siphoning the vapors back into the intake manifold so they can be reburned in the engine. Used since 1968 on most vehicles, the main component is a spring-loaded PCV valve that meters airflow. The PCV valve is usually mounted in a valve cover, and is connected to the intake manifold, carburetor or throttle body with a large vacuum hose. The intake vacuum pulls fresh air into the crankcase through a second breather hose. As the air passes through the engine, it picks up blowby vapors and moisture before it is pulled through the PCV valve and back into the engine. The PCV valve changes the flow rate according to engine load and throttle position.


PCV helps extend oil life by removing moisture that causes sludge. If the PCV valve or hose becomes clogged, moisture can rapidly accumulate in the crankcase causing engine-damaging sludge to form. A plugged PCV may also allow pressure to build inside the engine, which can cause oil leaks.

A quick check of the PCV valve is to shake it and listen for a rattle (no rattle would indicate a blockage). Another check is to pull the valve from the valve cover and feel the end for vacuum while the engine is idling (no vacuum would indicate a blockage). The recommended replacement interval is typically 50,000 miles.


Some engines do not use a PCV valve for crankcase ventilation, but they have a breather box that serves the same purpose.

On some older vehicles, a small PCV air filter is mounted inside the air cleaner. This filter should always be inspected and cleaned or replaced as needed on a regular basis.

EGR lowers oxides of nitrogen (NOX) in the exhaust by recirculating a small amount of exhaust into the intake manifold to reduce combustion temperatures. EGR is not used at idle, and only comes into play when the engine is under load or accelerating.

Most vehicles have a vacuum-operated EGR valve on the engine that opens a port between the intake and exhaust manifolds when intake vacuum drops and/or exhaust backpressure reaches a certain level (indicating the engine is under load). Some EGR systems also have solenoids or electronic EGR valves that provide increased control over EGR operation.


An inoperative EGR system or EGR valve that fails to open or is clogged with carbon will increase NOX emissions and usually cause engine-damaging detonation (spark knock or pinging) when accelerating. An EGR valve that is stuck open will have the same effect as a vacuum leak and cause lean misfire, rough idle and possible stalling.

Replacement EGR valves must be the correct one for the application. Calibration is very important because the flow characteristics of the valve must closely match engine requirements. Some aftermarket replacement EGR valves have various adapters to modify the flow rate.

These prevent the escape of fuel vapors from the fuel tank (and carburetor on older vehicles) into the atmosphere by trapping the vapors in a canister filled with activated charcoal. The vapors are stored until they can be siphoned back into the engine later and reburned. This is controlled by the canister purge valve. Other parts of the system include vapor hose connections and the fuel tank filler cap. On many 1995 and newer vehicles, the OBD II system performs an EVAP emissions test to check for vapor leaks. If leakage exceeds a certain rate, the Check Engine light may come on.


The EVAP system requires no maintenance. Most problems that occur are in the purge control valve or a leaky fuel filler cap. Replacement gas caps must be the same type as the original (caps for some older vehicles may be vented, but newer ones are sealed).


Some vehicles use a belt-driven air pump to pump extra oxygen into the exhaust system to reduce pollution. A diverter valve assembly on the air pump controls the flow of air into the exhaust manifold. On some vehicles, air is also routed to the catalytic converter. An anti-backfire valve prevents hot exhaust gases from flowing backwards into the system and damaging the diverter valve or air pump. Problems in this system can cause an increase in emissions.


The catalytic converter is an "afterburner" in the exhaust system that reduces pollution. Used since 1975 on all engines, the converter is located in the exhaust system just behind the engine. Some engines have two converters (one for each side of a V6 or V8 engine), and some have four (two additional converters in each exhaust manifold).

Inside the converter is a ceramic honeycomb or pellets coated with platinum, palladium and rhodium. These metals are catalysts and trigger chemical reactions that reduce unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOX) in the exhaust. The reactions produce a lot of heat, causing the converter to run extremely hot. Older cars mostly used "two-way" converters that reduce HC and CO only. Newer vehicles use "three-way" or "three-way plus oxygen" converters that also reduce NOX.


OEM converters are designed to last 100,000 to 150,000 miles under normal service. On 1995 and newer vehicles with OBD II, a second oxygen sensor is mounted behind the converter to monitor its operation. If converter efficiency drops below a certain level, it will turn on the Check Engine light.

The catalyst inside a converter can be contaminated by lead (leaded gasoline), silicon (coolant leaks) or phosphorus (oil burning), reducing its efficiency. Converters can also be damaged by overheating if an engine is misfiring, leaking compression or burning oil. Severe overheating can melt the inside of the converter, causing a blockage that restricts the exhaust. A plugged converter causes an increase in exhaust backpressure that may cause the engine to stall. There is no way to clean a plugged or contaminated converter, so replacement is the only option.


Aftermarket replacement converters must the be the same as the original, and installed in the same location as the original. It is illegal to remove the converter from a street-driven vehicle or to replace it with a straight pipe.

As part of the computer-controlled fuel delivery system, the O2 sensor monitors oxygen levels in the exhaust to indicate if the fuel mixture is running rich or lean. The computer looks at the O2 sensor signal, then rebalances the fuel mixture as needed to maintain the lowest possible emissions. This is called the "feedback fuel control loop" because the signal from the O2 sensor causes the fuel mixture to change.


The O2 sensor is mounted in the exhaust manifold (one on each side of a V6 or V8 engine). When the sensor gets hot (about 600 degrees), it starts to generate a voltage signal that changes when the fuel mixture is rich or lean. Most newer O2 sensors have an internal heater so that they will reach operating temperature more quickly (which reduces cold start emissions), and it prevents the sensor from cooling off when the engine is idling.

There are several basic types: unheated two-wire zirconium O2 sensors, heated two- or three-wire zirconium O2 sensors, and heated titania O2 sensors. Replacement sensors must be the same as the original. Some aftermarket O2 sensors require wires to be spliced when the sensor is replaced. Others come with connectors that are the same as the OEM sensor.


A bad O2 sensor will cause an increase in fuel consumption and emissions, and may cause the Check Engine light to come on. O2 sensors also become sluggish with age as contaminants build up on the sensor tip. Some OEMs have a recommended replacement interval for O2 sensors, but most do not.

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